1. Field of the Invention
The present invention relates to an optical system including fluoride.
2. Description of the Related Art
Optical systems such as optical lenses, displays, or optical waveguides include antireflection coatings for lessening a loss of light amount or the like due to reflection. Magnesium fluoride (MgF2) has been widely used as an antireflection material of a low refractive index. This is because i) MgF2 produces a high antireflection effect because of its low refractive index (1.38), ii) MgF2 can be easily formed into a film by vacuum evaporation, and iii) MgF2 has sufficient durability when formed on a surface of a substrate heated to about 300° C.
However, when the substrate is made of plastic or is provided with an electric circuit such as a semiconductor, it is impossible to heat the substrate to 300° C. or higher. Therefore, it is impossible to manufacture an optical system configured such that a magnesium fluoride film is formed on the substrate having such a low heat resistance. While Japanese Laid-Open Patent Application Publication No. HEI. 09-243802 discloses a method of depositing magnesium fluoride at low temperatures using special sputtering, a specific characteristic of the magnesium fluoride film is not clearly described there.
Japanese Laid-Open Patent Application Publication No. Hei. 10-90532 discloses an optical waveguide comprising a substrate having a low refractive index and provided with grooves on a main surface thereof, transparent resin having a high refractive index and filled in the grooves, and resin having a low refractive index and covering the main surface, and a manufacturing method thereof. In this optical waveguide, the substrate is made of PMMA (polymethylmethatcrylate) having a relatively high refractive index (1.49). Because a material having a refractive index higher than that of the PMMA substrate needs to be selected as the transparent resin to be filled in the grooves, and transparent resin that reacts with the substrate cannot be used, available transparent resin is limited.
A problem associated with a touch panel is that, in a device including an electronic display such as a liquid crystal display or a touch panel provided on the electronic display, ambient light from outside incident on a display surface of the device is reflected under bright environment, so that image contrast is reduced, and ambient environment is mirrored in the display surface, thereby resulting in degraded visibility. Under the circumstance, for the purpose of improving image contrast and visibility, an attempt has been made to prevent occurrence of surface reflection on the surface of the liquid crystal display or the surface of the touch panel provided on the surface of the liquid crystal display. As a method of preventing the surface reflection, there are two known methods in which, i) on a surface of a transparent film constituting a polarizer of the liquid crystal display, or on a transparent substrate on a surface side of the touch panel, a film having a refractive index lower than that of the transparent film or the transparent substrate is formed to have thickness equal to ¼ of a visible wavelength, for reducing reflection by interference effect, and ii) two or more types of layers having different refractive indices are formed for reducing the reflection in a wider wavelength range.
As method of forming the antireflection coating on the surface of the polarizer or the surface of the touch panel, there are methods in which i) fluoride such as magnesium fluoride is deposited to form the antireflection coating by vacuum evaporation or sputtering, and ii) a solution obtained by dissolving resin such as polymer containing fluorine with a low refractive index is coated and dried to form the antireflection coating (see Japanese Laid-Open Patent Application Publication No. Hei. 6-115023).
However, in the method of depositing fluoride to form the antireflection coating, a high temperature of 300° C. or higher is required to form the magnesium fluoride film that has high transmittance and high wear resistance, and hence, this method cannot be applied to the polarizer or the touch panel with low heat-resistance temperature. Besides, since the magnesium fluoride film deposited by vacuum evaporation, sputtering, or the like is generally polycrystalline having a column-shaped structure, it is porous as having a specific surface area which is greater than 10 m2/g. Therefore, the film has a drawback of high water absorbing ability. Specifically, since the amount of water absorbed in the magnesium fluoride film varies with variation in ambient temperature and humidity (moisture content), the refractive index of the film is shifted depending on environmental conditions. Further, it is necessary to deposit the film allowing for the environmental conditions and variation in the refractive index. Meanwhile, there is a method in which titanium oxide (TiO2) and silicon oxide (SiO2) which can be formed into films at low temperatures are alternately disposed, but in this method, five or more layers are required for achieving low reflectance in a wide wavelength range, which leads to reduced productivity.
The method of coating and drying the solution to form the antireflection coating is capable of forming the antireflection coating of large area with increased productivity, but the polymer containing fluorine has generally low hardness and low wear resistance. Accordingly, there has been proposed a method of applying a multifunctional polymeric monomer containing fluorine and then polymerizing and curing the monomer by electron beam irradiation (see Japanese Laid-Open Patent Application Publication No. Hei. 8-48935). This method offers a film comprised of a polymer containing fluorine that has hardness and improved wear resistance, but might damage the substrate or the like, for example, color the substrate, by the electron beam irradiation.